Method and apparatus for encoding video using granular downsampling of frame resolution

ABSTRACT

Disclosed herein are methods and apparatuses for encoding a video signal having a plurality of frames, each frame having a plurality of blocks and an original resolution. The method includes identifying a first frame from the plurality of frames that is predicted using inter prediction, determining a first variance for the first frame using a processor. selecting a first frame resolution from one of: (a) the original resolution or (b) a resolution lower than the original resolution for the first frame based on the first variance, and encoding the first frame using the selected first frame resolution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 13/096,285, which was filed on Apr. 28, 2011.

TECHNICAL FIELD

The present invention relates in general to video signal transmission and particularly to the encoding and decoding of such a signal.

BACKGROUND

An increasing number of applications today make use of digital video signals for various purposes including, for example, business meetings between people in remote locations via video conferencing, high definition video entertainment, video advertisements, and sharing of user-generated videos. As technology is evolving, users have higher expectations for video quality and resolution even when video signals are transmitted over communications channels having limited bandwidth.

To permit transmission of digital video streams while limiting bandwidth consumption, a number of video compression schemes have been devised, including formats such as VPx, promulgated by Google Inc. of Mountain View, Calif., and H.264, a standard promulgated by ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG), including present and future versions thereof. H.264 is also known as MPEG-4 Part 10 or MPEG-4 AVC (formally, ISO/IEC 14496-10).

These compression schemes can use quantization techniques on frames of a digital video stream to reduce the bitrate (i.e. data size) of the encoded digital video stream. These quantization techniques discard part of a frame's data using standard computations, thereby reducing the frame's bitrate. Although these quantization techniques reduce the bitrate, they may not suitably maintain the quality of the video signal.

SUMMARY

Disclosed herein are embodiments of methods and apparatuses for encoding a video signal.

One aspect of the disclosed embodiments is a method for encoding a video signal having a plurality of frames, each frame having a plurality of blocks and an original resolution. The method includes identifying a first frame from the plurality of frames that is predicted using inter prediction, determining a first variance for the first frame using a processor, selecting a first frame resolution from one of: (a) the original resolution or (b) a resolution lower than the original resolution for the first frame based on the first variance, and encoding the first frame using the selected first frame resolution.

Another aspect of the disclosed embodiments is a method for determining at least one threshold used for encoding a video signal having a plurality of frames, each frame having a plurality of blocks. The method includes identifying a test sequence of frames, the frames in the test sequence of frames having an original resolution, calculating a variance for at least one frame in the test sequence, calculating a first PSNR for the at least one frame using the original resolution, calculating a second PSNR for the at least one frame using a lower resolution that is less than the original resolution, and determining the at least one threshold using the variances, first PSNRs, and second PSNRs.

Another aspect of the disclosed embodiments is an apparatus for encoding a video signal having at least one frame, each frame having a plurality of blocks, each block having a plurality of pixels. The apparatus comprises a memory and at least one processor configured to execute instructions stored in the memory to: identify a first frame from the plurality of frames that is predicted using inter prediction, determine a first variance for the first frame using a processor, select a first frame resolution from one of: (a) the original resolution or (b) a resolution lower than the original resolution for the first frame based on the first variance, encode the first frame using the selected first frame resolution.

These and other embodiments will be described in additional detail hereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:

FIG. 1 is a schematic of a video encoding and decoding system;

FIG. 2 is a flowchart diagram of an exemplary method of determining a resolution for a frame in the system of FIG. 1;

FIG. 3 is a flowchart diagram of an exemplary method of determining a variance for a frame predicted by intra prediction usable in the method of FIG. 2;

FIG. 4 is a flowchart diagram of an exemplary method of determining a resolution for a frame predicted by intra prediction usable in the method of FIG. 2;

FIG. 5 is a flowchart diagram of an exemplary method of determining a variance for a frame predicted by inter prediction usable in the method of FIG. 2;

FIG. 6 is a flowchart diagram of an exemplary method of determining a resolution for a frame predicted by inter prediction usable in the method of FIG. 2; and

FIG. 7 is a flowchart diagram of an exemplary method of determining a threshold value usable in the method of FIG. 2.

DETAILED DESCRIPTION

This Application hereby incorporates by reference in the entirely the following: co-pending U.S. patent application Ser. No. 13/095,967 filed on Apr. 28, 2001 and entitled “METHOD AND APPARATUS FOR ENCODING VIDEO BY DETERMINING BLOCK RESOLUTION”, co-pending U.S. patent application Ser. No. 13/095,968 filed on Apr. 28, 2011 and entitled “METHOD AND APPARATUS FOR ENCODING VIDEO BY DOWNSAMPLING FRAME RESOLUTION”.

FIG. 1 is a diagram of an encoder and decoder system 10 for still or dynamic video images. An exemplary transmitting station 12 may be, for example, a computer having an internal configuration of hardware including a processor such as a central processing unit (CPU) 14 and a memory 16. CPU 14 can be a controller for controlling the operations of transmitting station 12. The CPU 14 is connected to memory 16 by, for example, a memory bus. Memory 16 may be random access memory (RAM) or any other suitable memory device. Memory 16 can store data and program instructions which are used by the CPU 14. Other suitable implementations of transmitting station 12 are possible.

A display 18 configured to display a video stream can be connected to transmitting station 12. Display 18 may be implemented in various ways, including by a liquid crystal display (LCD) or a cathode-ray tube (CRT). Display 18 may also be configured for other uses, such as screencasting. Alternatively, or in addition to display 18, a video stream can be generated from a video camera 20 or a received from a video file 22 and can be transferred to transmitting station 12.

A video stream can consist of a number of adjacent video frames (i.e. images), which may be still or dynamic. Adjacent video frames can be further subdivided into a single frame. At the next level, the frame can be divided into a series of blocks, which contain data corresponding to, for example, a 16×16 block of displayed pixels. Each block can contain luminance and chrominance data for the corresponding pixels. The blocks can also be of any other suitable size such as 16×8 pixel groups or 8×16 pixel groups. In other embodiments, video stream may only include a single frame and may be in applications such as screencasting.

A network 28 connects transmitting station 12 and a receiving station 30 for encoding and decoding of the video stream. Specifically, the video stream can be encoded by an encoder in transmitting station 12 and the encoded video stream can be decoded by a decoder in receiving station 30. Network 28 may, for example, be the Internet. Network 28 may also be a local area network (LAN), wide area network (WAN), virtual private network (VPN), or any other means of transferring the video stream from transmitting station 12.

Receiving station 30, in one example, may be a computer having an internal configuration of hardware include a processor such as a central processing unit (CPU) 32 and a memory 34. CPU 32 is a controller for controlling the operations of transmitting station 12. CPU 32 can be connected to memory 34 by, for example, a memory bus. Memory 34 may be RAM or any other suitable memory device. Memory 34 stores data and program instructions which are used by CPU 32. Other suitable implementations of receiving station 30 are possible.

FIG. 2 is a flowchart of an exemplary method of determining resolution for a frame in the system of FIG. 1. The method 50 allows for the encoder to determine in a granular fashion the resolution to encode each frame in a video stream. For example, the encoder may encode a group of pictures (GOP) that contains frames encoded at different resolutions. To contrast, some current encoding schemes only provide the ability to encode frames at different resolutions in intervals based on GOP.

The encoder first determines whether the current frame is an intra predicted frame (I-frame) (52). The current frame may be an I-frame based on, for example, its position in the digital video stream. The current frame can be an I-frame based on a number of alternative considerations that may or may not be used by the encoder. For example, the current frame could be an I-frame if requested by the receiving station 24, or if the pre-determined resolution is set. However, some encoding schemes may require that a GOP be of a fixed length. In these encoding schemes, the I-frame determination would be based on a number of frames since the last I-frame.

If the current frame is an I-frame, the encoder determines the intra prediction frame variance of the current frame (54). A method of determining an intra prediction frame variance is described in more detail later with respect to FIG. 3. Next, the encoder selects the frame resolution for encoding the current frame based on the variance of the current frame (56). A method of selecting the frame resolution for an intra predicted frame is described in more detail later with respect to FIG. 4.

Once the frame resolution of the current frame is selected, the I-frame is encoded using the selected frame resolution (58). The encoding process can be performed using various encoding schemes, including variations of various current video encoding schemes presently available. The encoding process may be performed in parallel to the remainder of method 50. For example, the encoding of stage 60 may be performed on one processor in a computer, whereas the other stages of method 50 may be performed on another processor. Such a scheme would allow for the encoding of a first frame while another instance of method 50 determines the resolution of a next frame. In other words, the processing of one instance of method 50 would transfer to another processor at stage 60, while the processing of a new instance of method 50 begins on the one processor. Once the current frame is encoded, the method 50 ends.

Returning to stage 52, if the current frame is not an I-frame (i.e. it is to be encoded using inter prediction as a P-frame), the encoder determines the inter prediction frame variance of the current frame (60). A method of determining a inter prediction frame variance is described in more detail later with respect to FIG. 5. Next, the encoder selects the resolution of the current frame based on the variance of the current frame (62). A method of selecting the frame resolution of an inter predicted frame is described in more detail later with respect to FIG. 6.

Once the resolution of the current frame is selected, the encoder encodes the current frame using the selected frame resolution (64). As described previously with respect to stage 58, the encoding of the current frame in stage 64 may be performed on one processor in a computer, whereas the other stages of method 50 may be performed on another processor. Such a scheme would allow for the encoding of a first frame while method 50 determines the resolution of a next frame. Once the current frame is encoded, the method 50 ends.

FIG. 3 is a flowchart diagram of an exemplary method 80 of determining a variance for a frame predicted by intra prediction usable in the method 50 of FIG. 2. The encoder first can initialize any variables needed to determine the I-frame variance (82). Variables can include, for example, those for an intra-prediction variance and an intra-prediction variance total. Next, the encoder determines if there are any blocks left to process in the current frame (84). If so, the encoder selects the next block in the current frame (86). Blocks can be selected in any order, including raster order. The encoder then calculates the intra-prediction variance for the selected block (88).

The intra-prediction variance can be calculated by performing intra prediction on the blocks in the current frame. Intra prediction can be based on previously coded image samples within the current frame. Intra prediction can be performed on a current block by, for example, copying pixels (or filtered pixels) from adjacent, previously coded blocks to form a predicted block. The manner in which the pixels are copied can be by vertical prediction, horizontal prediction, DC prediction, True Motion prediction, southwest prediction, southeast prediction, vertical right diagonal prediction, vertical left diagonal prediction, horizontal down prediction, horizontal up prediction, etc.

Intra prediction can also be performed using a technique other than copying pixel values. For example, a predicted block can be formed for a current block using one or more parameterized equations. These parameterized equations can be, for example, an expression representing a curve that has a “best fit” to a defined set of previously coded pixels in the frame. Other techniques of determining a predicted block using intra prediction are also possible.

A residual block is determined based on the difference between the predicted block and the best-matching block. The intra-prediction variance can then be calculated using the below equations:

$\begin{matrix} {{{mean} = \frac{\sum\limits_{i = 0}{\sum\limits_{j = 0}p_{i,j}}}{N}};{wherein}} & (1) \end{matrix}$ i is a x-coordinate within the residual block; j is a y-coordinate within the residual block; p_(i,j) is a value of a pixel located at the coordinates of i, j within the residual block; and N is a number of pixels within the residual block. In addition,

$\begin{matrix} {{{intra} - {{prediction}\mspace{14mu}{variance}}} = \frac{\sum\limits_{i = 0}{\sum\limits_{j = 0}{{p_{i,j} - {mean}}}}}{N}} & (2) \end{matrix}$

The mean for the residual block is first calculated by averaging the values of all pixels within the residual block. The intra-prediction variance is then calculated by averaging the absolute value of the difference of each pixel from the mean of the residual block. The calculations above are exemplary only, and other similar means of determining the intra-prediction variance may be utilized.

The encoder next adds the calculated intra-prediction variance for the selected block to the intra-prediction variance total for the current frame (90). The encoder then returns to determine whether additional blocks are available within the current frame (stage 84). Once there are no blocks left to process, the encoder then normalizes the intra-prediction variance total (92). Normalization is used to equalize the scale of the intra-prediction variance total with the intra threshold that it will be later compared with. For example, the intra threshold may be of a per-block scale. In such a case, the intra-prediction variance total would be normalized by dividing it by the number of blocks in the current frame, and that result would be used as the frame's variance. In another example, the intra threshold may be of a per-frame scale. In such a case, the intra-prediction variance total would be normalized by leaving the intra-predicting variance total as-is and using it directly as the frame's variance.

FIG. 4 is a flowchart diagram of an exemplary method 100 of determining a resolution for a frame predicted by intra prediction usable in the method 50 of FIG. 2. The encoder will determine whether the current frame's intra-prediction variance (its variance) is greater than an intra threshold (102). The intra threshold is a pre-determined value than can be calculated using the method described later with respect to FIG. 7. If the variance is greater than the threshold, the current frame (I-frame) is encoded at a resolution less than the original resolution (104). Otherwise, no change is made to the current frame's resolution (106). In other words, the frame is encoded at its original resolution.

FIG. 5 is a flowchart diagram of an exemplary method 120 of determining a variance for a frame predicted by inter prediction usable in the method 50 of FIG. 2. The encoder first can initialize any variables needed to determine the P-frame variance (122). Variables can include, for example, those for an inter-prediction variance, intra-prediction variance, and a variance total. Next, the encoder determines if there are any blocks left to process in the current frame (124). If so, the encoder selects the next block in the current frame (126). Blocks can be selected in any order, including raster order. The encoder then calculates the intra-prediction variance and the inter-prediction variance for the selected block (128).

As described before, the intra-prediction variance can be calculated by copying pixel values from previously coded blocks, using a parameterized equation or any other possible technique. A residual block is determined based on the difference between the predicted block and the best-matching block. The intra-prediction variance can then be calculated using equations (1) and (2) above or by an equivalent set of calculations.

The inter-prediction variance can be calculated by first performing an inter-frame motion vector search for a best-matching block in a reference frame. The reference frame can be any reference frame available in the encoding scheme used, including, for example, a last frame, a last I-frame, or an alternative reference frame. A residual block is determined based on the difference between the current block and the best-matching block. A motion vector is also encoded that describes the position of the best-matching block relative to the position of the current block. The inter-prediction variance can then be calculated using equations (1) and (2) above or by an equivalent set of calculations.

The encoder next adds one of the calculated inter-prediction variance or the intra-prediction variance to the variance total (130). In one embodiment, the inter-prediction variance is added to the variance total. In another embodiment, the variance added to the variance total is based on which variance is the smallest variance. The choice between the smallest of the inter-prediction variance and the intra-prediction variance can be done because an inter-predicted frame may contain blocks that are both inter predicted and intra predicted. In this case, the encoder may take this into account by using the intra-prediction variance if it finds a better matching block than can be found using inter prediction.

The encoder then returns to determine whether additional blocks are available within the current frame (stage 124). Once there are no blocks left to process, the encoder then normalizes the variance total (122). The normalization process can be the same or similar process as described previously with respect to stage 92. The normalized variance total is the inter variance of the current frame.

FIG. 6 is a flowchart diagram of an exemplary method 140 of determining a resolution for a frame predicted by inter prediction usable in the method 50 of FIG. 2. The encoder will determine whether the current frame's inter variance is greater than an inter threshold (142). The inter threshold is a pre-determined value than can be calculated using the method described later with respect to FIG. 7.

If the variance is not greater than the threshold, the encoder will perform inter prediction using a reference frame at the original frame resolution and will create a residual frame at the original resolution (144). If necessary, the reference frame will be up-sampled to the original resolution. In other words, if the reference frame of the current frame is encoded at a resolution less than the original resolution, reference frame will be up-sampled to the original resolution before inter-prediction is performed.

Returning to stage 142, f the inter variance is greater than the inter threshold, the encoder checks the resolution of the reference frame (146). If the reference frame is at the original resolution, then the inter prediction will be performed at the original resolution, and the resulting residual will be down-sampled to a resolution less than the original resolution for encoding (148). Otherwise, if the reference frame is at a resolution less than the original resolution, then the inter prediction will be performed at that lower resolution, and the resulting residual will be encoded at that lower resolution (150). In other words, the inter prediction will be performed at the reference frame resolution, and the residual output will be adjusted to a different resolution if necessary.

FIG. 7 is a flowchart diagram of an exemplary method 160 of determining a threshold value usable in the method 50 of FIG. 2. The method 160 is capable of determining either an intra threshold or an inter threshold. Each of the intra and inter thresholds can be a pre-determined value that are each determined by method 160 and then included in the encoder. To determine a threshold using method 160, first, any variables needed to determine the threshold can be initialized (102). Such variables can include, for example, one or more arrays for PSNR_O, PSNR_L, and frame variance, which will be discussed in more detail below.

The method 160 operates on a test sequence of frames. The test sequence of frames can contain video data similar to that expected to be encoded. For example, in a screen casting encoding application, the test sequence of frames could be an exemplary screen casting video data stream. In another example, if an encoder could be used for screen casting and for encoding of moving pictures (i.e. video clips and/or movies), the test sequence of frames can include both a screen casting video data stream and a moving picture video data stream. The test sequence of frame can also be based on video data from other sources.

Once variables are initialized, the method 160 next includes checks to see if any frames are left to process in the test sequence of frames (164). If there is at least one frame left, the next frame for processing is selected (166). The variance of the selected frame is calculated using a method such as method 80 of determining an intra frame variance or method 120 of determining an inter frame variance (168). The selected frame is encoded and then decoded using its original resolution (170). The encoding is performed to create an encoded frame that is within a target bitrate. If the threshold being calculated is the inter threshold, the encoding is performed using inter prediction, whereas if the threshold is the intra threshold, the encoding is performed using intra prediction.

An original resolution peak signal-to-noise-ratio (PSNR_O) will be calculated using the frame and the decoded frame (172). A PSNR value is a measure of quality comparing the original frame and a lossy-encoded reconstructed (decoded) frame. In this case, the PSNR_O measures the quality of the resulting decoded frame after being compressed to the target bitrate using techniques other than the changing of pixel resolution (i.e. quantization).

The PSNR can be calculated using a mean squared error (MSE). The PSNR alternatively can be calculated using other means. One exemplary equation for calculating the MSE and PSNR is provided:

$\begin{matrix} {{{MSE} = {\frac{1}{mn}{\sum\limits_{i = 0}^{m - 1}{\sum\limits_{j = 0}^{n - 1}\left\lbrack {{S\left( {i,j} \right)} - {D\left( {i,j} \right)}} \right\rbrack^{2}}}}};{wherein}} & (3) \end{matrix}$ i is a x-coordinate; j is a y-coordinate; S is the selected frame; D is the decoded frame; m is the width of the frames S and D; n is the height of the frames S and D. D is the decoded frame. In addition,

$\begin{matrix} {{{PSNR} = {10\;{\log_{10}\left( \frac{{MAX}_{S}^{2}}{MSE} \right)}}};{wherein}} & (4) \end{matrix}$ wherein MAX_(s) is the maximum possible pixel value of the selected frame.

Once the PSNR_O has been calculated, the selected frame will be downsampled to a resolution less than the original resolution, the downsampled frame will be encoded, the encoded downsampled frame will then be decoded, and the decoded downsampled frame will then be upsampled to the original resolution (174). As with the encoding of the original resolution frame, the encoding of the downsampled frame is performed using the target bitrate. Also, if the threshold being calculated is the inter threshold, the encoding is performed using inter prediction, whereas if the threshold is the intra threshold, the encoding is performed using intra prediction. The purpose is to create a decoded upsampled frame for comparison with the selected frame. The resolution of the downsampled frame can be determined using one or more pre-determined lower resolutions. Alternatively, the resolution of the downsampled frame can be determined on a frame-by-frame basis, selected by a user or any other technique.

A lower resolution peak signal-to-noise-ratio (PSNR_L) is then calculated (176). In this case, the PSNR_L measures the quality of the resulting decoded upsampled frame after being compressed to the target bitrate using the technique of changing the pixel resolution.

Once the variance, PSNR_O, and PSNR_L have been calculated for the selected frame, the method 160 returns to stage 164 to determine if any additional frames are available in the test sequence of frames. Once there are no frames left, the method 160 includes plotting the variance, PSNR_O, and PSNR_L values calculated for each frame (178). The plot includes two series of data. The first series includes the variance for each frame versus the PSNR_O value for each frame. The second series includes the variance for each frame versus the PSNR_L value for each frame.

The first and second series can be plotted using fitted curve techniques. For example, an approximate fitted curve function can be determined to approximate each series. The fitted curve techniques used can include techniques such as the least squares method. Alternatively, the first and second series can be plotted using their actual values. Plotting may not involve the actual placement of data points on a coordinate plane. Rather, plotting may merely be an intermediate step performed by a processor.

Next, an intersection between the first series and the second series is determined (180). The intersection may be determined computationally by a processor based on the fitted curves determined for each series. But the intersection can also be determined using other methods. For example, a programmer or other person may select the intersection based on a plot of each series on a coordinate plane. The selected intersection is the threshold (182). Alternatively, the selected intersection's value may be multiplied by a constant or processed by a standard function to normalize it for use in the encoder as the threshold.

The operation of encoding or decoding can be performed in many different ways and can produce a variety of encoded data formats. The above-described embodiments of encoding or decoding may illustrate some exemplary encoding techniques. However, in general, encoding and decoding are understood to include any transformation or other change of data whatsoever.

The embodiments of transmitting station 12 and/or receiving station 30 (and the algorithms, methods, instructions etc. stored thereon and/or executed thereby) can be realized in hardware, software, or any combination thereof including, for example, IP cores, ASICS, programmable logic arrays, optical processors, programmable logic controllers, microcode, firmware, microcontrollers, servers, microprocessors, digital signal processors or any other suitable circuit. In the claims, the term “processor” should be understood as encompassing any the foregoing, either singly or in combination. The terms “signal” and “data” are used interchangeably. Further, portions of transmitting station 12 and receiving station 30 do not necessarily have to be implemented in the same manner.

Further, in one embodiment, for example, transmitting station 12 or receiving station 30 can be implemented using a general purpose computer/processor with a computer program that, when executed, carries out any of the respective methods, algorithms and/or instructions described herein. In addition or alternatively, for example, a special purpose computer/processor can be utilized which can contain specialized hardware for carrying out any of the methods, algorithms, or instructions described herein.

Transmitting station 12 and receiving station 30 can, for example, be implemented on computers in a screencasting system. Alternatively, transmitting station 12 can be implemented on a server and receiving station 30 can be implemented on a device separate from the server, such as a hand-held communications device (i.e. a cell phone). In this instance, transmitting station 12 can encode content using an encoder into an encoded video signal and transmit the encoded video signal to the communications device. In turn, the communications device can then decode the encoded video signal using a decoder. Alternatively, the communications device can decode content stored locally on the communications device (i.e. no transmission is necessary). Other suitable transmitting station 12 and receiving station 30 implementation schemes are available. For example, receiving station 30 can be a personal computer rather than a portable communications device.

Further, all or a portion of embodiments of the present invention can take the form of a computer program product accessible from, for example, a computer-usable or computer-readable medium. A computer-usable or computer-readable medium can be any device that can, for example, tangibly contain, store, communicate, or transport the program for use by or in connection with any processor. The medium can be, for example, an electronic, magnetic, optical, electromagnetic, or a semiconductor device. Other suitable mediums are also available.

The above-described embodiments have been described in order to allow easy understanding of the present invention and do not limit the present invention. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structure as is permitted under the law. 

What is claimed is:
 1. A method for determining at least one threshold used for encoding a video signal having a plurality of frames, each frame having a plurality of blocks, the method comprising: identifying a test sequence of frames, the frames in the test sequence of frames having an original resolution; calculating a variance for at least one frame in the test sequence; calculating a first PSNR for the at least one frame using the original resolution; calculating a second PSNR for the at least one frame using a lower resolution that is less than the original resolution; and determining the at least one threshold using the variances, first PSNRs, and second PSNRs by identifying a first series comprising the variances and the first PSNRs, identifying a second series comprising the variances and the second PSNRs, determining an intersection between the first series and the second series, and determining the threshold using the intersection.
 2. The method of claim 1, further comprising: including the at least one threshold in an encoder; and configuring the encoder to use the at least one threshold to select a frame resolution for at least one of the frames that it can encode.
 3. The method of claim 1, wherein calculating the first PSNR for the at least one frame comprises: selecting a frame from the test sequence of frames; encoding the selected frame; decoding the encoded frame; and calculating the first PSNR using the selected frame and the decoded frame.
 4. The method of claim 3, wherein encoding the selected frame uses a target bitrate.
 5. A method for determining at least one threshold used for encoding a video signal having a plurality of frames, each frame having a plurality of blocks, the method comprising: identifying a test sequence of frames, the frames in the test sequence of frames having an original resolution; calculating a variance for at least one frame in the test sequence; calculating a first PSNR for the at least one frame using the original resolution by selecting a frame from the test sequence of frames, encoding the selected frame, decoding the encoded frame, and calculating the first PSNR using the selected frame and the decoded frame; calculating a second PSNR for the at least one frame using a lower resolution that is less than the original resolution by selecting a frame from the test sequence of frames, determining a downsampled frame at the lower resolution from the selected frame, encoding the downsampled frame, decoding the encoded downsampled frame, determining a decoded upsampled frame at the original resolution using the decoded downsampled frame, and calculating the second PSNR using the selected frame and the decoded upsampled frame; and determining the at least one threshold using the variances, first PSNRs, and second PSNRs.
 6. The method of claim 5, wherein the at least one threshold is an intra threshold, the variance is an intra-prediction variance, and each of encoding the selected frame and encoding the downsampled frame are performed using intra prediction.
 7. The method of claim 5, wherein the at least one threshold is an inter threshold, the variance is determined from one or more inter-prediction variances of blocks in the selected frame and one or more intra-prediction variances of blocks in the selected frame, and each of encoding the selected frame and encoding the downsampled frame are performed using inter prediction.
 8. The method of claim 5, wherein encoding the downsampled frame uses a target bitrate.
 9. The method of claim 1, wherein determining the intersection between the first series and the second series comprises: determining a first fitted curve from the first series; determining a second fitted curve from the second series; and determining the intersection as a value at which the first fitted curve and the second fitted curve intersect.
 10. An apparatus for determining at least one threshold used for encoding a video signal having a plurality of frames, each frame having a plurality of blocks, the apparatus comprising: a memory having computer-executable instructions stored therein; and at least one processor configured to execute the instructions, wherein the instructions, when executed by the at least one processor, cause the at least one processor to: identify a test sequence of frames, the frames in the test sequence of frames having an original resolution; calculate a variance for at least one frame in the test sequence; calculate a first PSNR for the at least one frame using the original resolution; calculate a second PSNR for the at least one frame using a lower resolution that is less than the original resolution; and determine the at least one threshold using the variances, first PSNRs, and second PSNRs by identifying a first series comprising the variances and the first PSNRs, identifying a second series comprising the variances and the second PSNRs, determining an intersection between the first series and the second series, and determining the threshold using the intersection.
 11. The apparatus of claim 10, wherein the instructions further cause the one or more processors to: include the at least one threshold in an encoder; and configure the encoder to use the at least one threshold to select a frame resolution for at least one of the frames that it can encode.
 12. The apparatus of claim 10, wherein the at least one processor calculates the first PSNR for the at least one frame by: selecting a frame from the test sequence of frames; encoding the selected frame; decoding the encoded frame; and calculating the first PSNR using the selected frame and the decoded frame.
 13. The apparatus of claim 12, wherein encoding the selected frame uses a target bitrate.
 14. An apparatus for determining at least one threshold used for encoding a video signal having a plurality of frames, each frame having a plurality of blocks, the apparatus comprising: a memory having computer-executable instructions stored therein; and at least one processor configured to execute the instructions, wherein the instructions, when executed by the at least one processor, cause the at least one processor to: identify a test sequence of frames, the frames in the test sequence of frames having an original resolution; calculate a variance for at least one frame in the test sequence; calculate a first PSNR for the at least one frame using the original resolution by selecting a frame from the test sequence of frames, encoding the selected frame, decoding the encoded frame, and calculating the first PSNR using the selected frame and the decoded frame; calculate a second PSNR for the at least one frame using a lower resolution that is less than the original resolution by selecting a frame from the test sequence of frames, determining a downsampled frame at the lower resolution from the selected frame, encoding the downsampled frame, decoding the encoded downsampled frame, determining a decoded upsampled frame at the original resolution using the decoded downsampled frame, and calculating the second PSNR using the selected frame and the decoded upsampled frame; and determine the at least one threshold using the variances, first PSNRs, and second PSNRs.
 15. The apparatus of claim 14, wherein the at least one threshold is an intra threshold, the variance is an intra-prediction variance, and each of encoding the selected frame and encoding the downsampled frame are performed using intra prediction.
 16. The apparatus of claim 14, wherein the at least one threshold is an inter threshold, the variance is determined from one or more inter-prediction variances of blocks in the selected frame and one or more intra-prediction variances of blocks in the selected frame, and each of encoding the selected frame and encoding the downsampled frame are performed using inter prediction.
 17. The apparatus of claim 14, wherein encoding the downsampled frame uses a target bitrate.
 18. The apparatus of claim 10, wherein determining the intersection between the first series and the second series comprises: determining a first fitted curve from the first series; determining a second fitted curve from the second series; and determining the intersection as a value at which the first fitted curve and the second fitted curve intersect. 